1. Field of the Invention
The present invention is directed to a semiconductor chip carrier assembly and a process for making that assembly. More particularly, the present invention is directed to a chip carrier assembly utilizing a microporous film adhesive to bond the flexible circuit and the stiffener members.
2. Field of the Prior Art
It is well known in the art to electrically connect a semiconductor chip to a circuit board by employing a carrier structure for the semiconductor chip. One well accepted chip carrier assembly design for providing this electrical connection is a tape ball grid array type assembly (TBGA). A TBGA assembly is electrically connected to a circuit board by means of solder balls. This electrical connection is accomplished by connecting the solder balls of the chip assembly to electrical connecting pads on the circuit board.
The aforementioned assembly requires the inclusion of a metal stiffener to provide adequate structural support for the very thin polyimide tape which constitutes the flexible circuit substrate.
A chip carrier assembly stiffener is bonded to the polyimide tape substrate by means of an adhesive. One commonly employed adhesive is fiberglass reinforced pressure sensitive acrylic that acts not only as an adhesive but also as an electric insulating barrier to prevent solder rivets from electrically communicating with the stiffener. Unfortunately, acrylic film adhesives, although somewhat conformable, do not entirely conform around the protruding solder rivets. As such, the use of acrylic film adhesives oftentimes result in the formation of pockets of air around the base of the rivets. Although this film adhesive is designed to overcome this problem, insofar as it is characterized by an embossed pattern to channel out air during assembly, air may still be entrapped between the flexible circuit and the adhesive.
The entrapment of air between the flexible circuit and the adhesive serves as a channel which permits the introduction of moisture. As those skilled in the art are aware, moisture causes corrosion failures and/or delamination during solder reflow. To avoid this, semiconductor chip assemblies that employ pressure sensitive acrylic film adhesives include the further processing step, prior to solder reflow, of baking the assembly.
There are additional detrimental aspects relating to the use of pressure sensitive film acrylic adhesives in chip carrier assemblies. Upon curing of the acrylic based adhesive, benzene vapor is produced as a by-product. This outgassing of benzene, a well-established carcinogen, represents an important health hazard.
Another adverse feature of commonly employed pressure sensitive film acrylic adhesives is the inclusion therein of metal oxide particles. The presence of metal oxide particles in acrylic adhesives represents a reliability hazard insofar as it can potentially lead to shorting out of the assembly.
Yet another detrimental result of employing pressure sensitive acrylic film adhesives is their tendency to oxidize upon exposure to long term temperature aging. Exposure of the assembly to this high temperature for this long duration leads to oxidation problems and has a long term detrimental effect upon the chip assembly.
U.S. Pat. No. 5,512,360 describes a prepreg adhesive composite formed from a porous polymeric substrate providing good adhesion, low dielectric constant and low modulus. The composite comprises at least one layer of an expanded polytetrafluoroethylene having an initial void volume of at least 50% to 95% and containing between 15% and 40% by weight, based on the total weight of the composite, of adhesive to form an uncured prepreg that eliminates or substantially reduces adhesive on the exterior, outer surfaces of the expanded polymeric structure. The composite of this patent is employed to make printed circuit boards as well as microwave substrates.